1. Field of the Invention
This invention relates to an automatic stroke adjustment device for a brake actuator.
2. Description of the Related Art
A brake actuator with an automatic stroke adjustment device at a cylinder bore opening is disclosed in U.S. Pat. No. 4,852,702 (Japanese provisional patent publication number 63-285341). FIG. 13 generally illustrates the automatic stroke adjustment device of the ""702 patent.
The automatic stroke adjustment device of the ""702 patent comprises a piston 21 slidably engaged with a cylinder bore 11a in a cylinder body 11 of a brake actuator 10; an adjustment nut 22 rotatably engaging a hollow in a top end of the piston 21 and abutting a stepped surface near the opening of the cylinder bore 11a; an adjustment bolt 30 having an externally threaded step 30a threadedly engaged with the internally threaded adjustment nut 22 so as to be non-rotatable with respect to the adjustment nut under an axial thrust, and having a bifurcated portion 30b at its top end holding a brake shoe 1 so as to restrict a rotation of the adjustment bolt 30; a clutch ring 40 having a clutching surface 40a making a clutch engagement with a clutching surface 11b at the opening side of the cylinder bore 11a, and having an internal thread 40b threadedly engaged with an external thread 22b on the adjustment nut 22 with an axial backlash between the two; and an adjustment spring 50 biasing the clutch ring 40 into engagement with the clutching surface 11b. Hereafter, xe2x80x9cscrew engagement threadedly engaged between the internal and external threads so as to be non-rotatable under an axial thrustxe2x80x9d may be called xe2x80x9cirreversible screw engagement,xe2x80x9d while xe2x80x9cscrew engagement threadedly engaged between the internal and external threads so as to be rotatable under an axial thrustxe2x80x9d may be called xe2x80x9creversible screw engagement.xe2x80x9d
Automatic stroke adjustment function of the conventional device is explained next. If the stroke of the piston 21 during operation of the brake actuator 10 is within a predetermined range, the adjustment nut 22 and the adjustment bolt 30 advance with the piston 21 within the backlash between the clutch ring 40 and the adjustment nut 22 and the clutch ring remains in clutching engagement with the clutching surface 11b. If, for example, a lining on a brake shoe 1 (not shown in the figure) wears and the stroke of the piston 21 exceeds the predetermined range, the adjustment nut 22 moves with the piston beyond the backlash between the adjustment nut 22 and the clutch ring 40. As a result of the force exerted on the clutch ring by the adjustment nut, the clutch engagement between the clutching surface 40a of the clutch ring 40 and a clutching surface 11b of the cylinder body 11 is released. The clutching ring 40, under the force of the adjustment spring 50, then slides on the clutching surface 11b of the cylinder body 11 and rotates along the external thread 22b on the adjustment nut 22.
When, the operation of the brake actuator 10 is released and the piston 21 returns due to a spring force by the shoe return spring, the adjustment nut 22 and the adjustment bolt 30 backstroke with the piston 21 across the backlash between the adjustment nut 22 and the adjustment bolt 30 without rotation and the clutch ring 40 engages the clutching surface 11b of the cylinder body 11 due to the spring force by the adjustment spring 50, thereby restricting relative rotation between the clutch ring and the clutching surface.
As the piston 21 continues to backstroke, until the stepped surface of the adjustment nut 22 engages the supporting section of the stepped surface of the cylinder bore 11a to restrict its rotation, the adjustment nut 22 rotates along the internal thread 40b of the clutch ring 40 as it backstrokes with the piston 21. The rotation of the adjustment nut 22 with respect to the adjustment bolt 30 screws the adjustment bolt 30 outward to adjust the stroking amount of the brake actuator 10 in response to the brake lining wear.
A brake actuator with an automatic stroke adjustment device at a bottom portion of a cylinder bore is disclosed in U.S. Pat. No. 5,713,437 (Japanese provisional patent publication number 9-229115). Particular sections of the automatic stroke adjustment device of the ""437 patent will be explained with reference to FIGS. 14 and 15.
This automatic stroke adjustment device for a brake actuator 110 comprises a piston 121 slidably engaged with a cylinder bore 11a in a cylinder body 111 of a brake actuator 110; an adjustment bolt 130 having an externally threaded stem 130c at one end making irreversible screw engagement with an internal thread 121c in a hollow 121b on the back side of the piston 121, and having a conical surface 130d at the other end making a clutch engagement with a first conical surface 111d formed on the partition wall 111c in the cylinder bore 111a; a clutch ring 140 having an internal thread 140b in reversible screw engagement with an external thread 130e formed on a large diameter axle portion of the adjustment bolt 130 with a backlash and a clutching surface 140a on the peripheral surface making a clutch engagement with a second conical surface 111e of the partition wall 111c; and an adjustment spring (drive ring spring) 150 biasing the clutch ring 140 to make a clutch engagement with the second conical surface.
A piston head 123 is restricted in its own rotation by holding and securing the brake shoe 1 in a bifurcated portion 123a formed at a top of the piston head 123, and the piston 121 is designed to be rotatable relative to the piston head 123 and the cylinder bore 111a. 
Automatic stroke adjustment operation is explained next. If the stroke of the piston 121 during the operation of the brake actuator 110 is within a predetermined range, the adjustment bolt 130 advances with the piston 121 within the backlash between the clutch ring 140 and the adjustment bolt 130 and the clutch ring 140 remains in clutching engagement with the second conical surface 111e. 
If a lining on a brake shoe 1 (not shown in the figure) wears and the stroke of the piston 121 exceeds the predetermined range, the adjustment bolt 130 moves beyond the backlash between the adjustment bolt 130 and the clutch ring 140. As a result, as the piston 121 continues to move the clutch engagement between the clutching surface 140a on the peripheral surface of the clutch ring 140 and the second conical surface 111e is released, and the clutch ring 140 under the force of the adjustment spring 150 slides on the second conical surface 111e of the partition wall 111c and rotates with respect to the adjustment bolt 130 along the external thread 130e on the adjustment bolt 130.
When, the operation of the brake actuator 110 is released and the piston 121 backstrokes to return due to a spring force by the shoe return spring, the piston 121 and the adjustment bolt 130 backstroke across the amount of backlash at the reversible screw engagement without the rotation of the piston 121 or the adjustment bolt 130, and the clutch ring 140 makes the clutch engagement with the second conical surface 111e of the partition wall 111c due to the spring force by the adjustment spring 150, thereby restricting the clutch ring""s rotation.
As the piston 121 continues to backstroke, until the conical surface 130d of the adjustment bolt 130 engages with the first conical surface 111d on the partition wall 111c of the cylinder bore 111a to restrict its rotation, the adjustment bolt 130 rotates along the internal thread 140b of the clutch ring 140. In response to the rotation of the adjustment bolt 130, the piston 121 is screwed outward to adjust the stroking amount of the brake actuator 110 in compensation for the brake lining wear.
Both of the above types of clutch rings 40 and 140 have lead angle internal threads 40b, 140b on their inner circumferential surfaces and need strength to rotate objects making irreversible screw engagement with another member under the spring force of the shoe return spring. In order to attain the above object, the conventional clutch rings 40, 140 are made of relatively thick plates, manufactured such as by machining or a combination of forging and machining.
However, the above-described conventional automatic stroke adjustment device for a brake actuator has the following drawbacks.
If the clutch ring is manufactured from a thick metal plate by all machining or combination of forging and machining, this method not only increases the material cost and manufacturing cost but also increases a weight of the device. Also, in consideration of the processability of the clutch ring such as by machining and forging, if soft materials such as brass are employed, there is a possibility of causing friction with an abutting surface of the adjustment spring, which reduces the durability. In order to improve the durability, a hard thin metal plate may be put between the adjustment spring and the surface abutting the adjustment spring. However, this method not only increases the number of components but also increases the manpower required to manufacture and assemble the device. Accordingly, employing the relatively thick plate has disadvantage in its cost and weight.
Further, the clutch ring may be a monoblock made by a sintered metal. However, this method not only increases the cost but also complicates the structure of the die, which reduces the processability.
An automatic stroke adjustment device must maintain its operation even if the brake drum experiences thermal expansion because of a frequent use on a long downhill. When the drum expands at elevated temperatures, the device adjusts the stroke to compensate. When the brake drum returns to normal temperatures, a clearance between the brake drum and the brake shoe reduces as a result of thermal contraction of the brake drum. To compensate for this contraction, the clearance under the normal temperature needs to be preset larger than optimal, which necessarily sacrifices a brake operation stroke.
This invention is intended to remove the aforementioned drawbacks, and an object of this invention is to provide an automatic stroke adjustment device for a brake actuator with an excellent improved processibility and a reduced weight. Further, another object of this invention is to provide an automatic stroke adjustment device for a brake actuator enabling one to stop the adjustment operation under high temperatures and to set a small shoe clearance under normal temperatures, thereby reducing the brake operation stroke.
The present invention is directed toward an automatic stroke adjustment device for a brake actuator, comprising a piston slidably fitting in a cylinder bore of a brake cylinder, a first screw engagement mechanism including a clutch ring and a first responsive member and allowing a relative rotation of the two due to an axial thrust, and a second screw engagement mechanism including the first responsive member and a second responsive member and inhabiting a relative rotation of the two under the axial thrust, in which if the piston overstrokes, a movement of the clutch ring of the first screw engagement mechanism exceeds a predetermined axial backlash between a female screw of the clutch ring and a male screw of the first responsive member, and a clutch engagement between the clutch ring and a third responsive member is released to allow a rotation of the clutch ring; if the overstroked piston then makes a back-stroke, when the backlash is filled up, the clutch engagement between the clutch ring and the third responsive member is activated making the two non-rotatable, and the first responsive member, making a screw engagement with the clutch ring, rotates and the second responsive member, making a screw engagement with the first responsive member, screws out until the first responsive member engages with a supporting member making the first responsive member non-rotatable, wherein the clutch ring is a monoblock ring formed by pressing a thin plate.
Another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator according to the first embodiment, wherein the first responsive member is an adjustment nut relatively rotatably fitting with the hollow at a top side of the piston and engaging with the supporting member at an opening side of the cylinder bore, the second responsive member is an adjustment bolt making a screw engagement with the female screw at an axis of the adjustment nut while the relative rotation therebetween due to an axial thrust is inhibited and the rotation is inhibited by a rotation regulator, a clutch ring makes a screw engagement with a male screw on a peripheral surface of the adjustment nut with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes the clutch engagement with the supporting member at the opening side of the cylinder bore, and an adjustment spring energizes the clutch ring in a direction to make a clutch engagement with the supporting member.
Yet another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator, wherein the first responsive member is the piston, the second responsive member is an adjustment bolt making a screw engagement with the female screw at an axis of the piston, while the relative rotation therebetween due to an axial thrust is inhibited and the rotation is inhibited by a rotation regulator, a clutch ring makes a screw engagement with a male screw on a peripheral surface at a tip end of the piston with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes the clutch engagement with the supporting member at the opening side of the cylinder bore, and an adjustment spring energizes the clutch ring in a direction to make a clutch engagement with the supporting member.
Still another embodiment of the present invention is directed toward an automatic stroke adjustment device for a brake actuator, wherein the second responsive member is the piston being inhabited the rotation by a rotation regulator, the first responsive member is an adjustment bolt, one end of which has a male screw stem making a screw engagement with a female screw axially formed at a back end side of the piston while the relative rotation between the two due to an axial thrust is inhibited, and the other end of which makes a clutch engagement with a bottom of the cylinder bore, a clutch ring makes a screw engagement with a male screw formed adjacent to a clutch section of the adjustment bolt with an axial backlash therebetween while the relative rotation therebetween due to an axial thrust is allowed and makes a clutch engagement with the bottom of the cylinder bore; and an adjustment spring energizes the clutch ring in the direction to make a clutch engagement with the bottom of the cylinder bore.
One particular embodiment of the present invention is directed toward a brake cylinder, wherein the clutch ring is made of a thermoreacting material which releases the clutch engagement of the clutch ring when it reaches a predetermined temperature in order to stop an automatic adjustment operation.
According to embodiments of the present invention, a thin plate may be pressed to form the clutch ring, which reduces the cost of material and machining. An economic advantage increases with the mass production of the automatic stroke adjustment device. Further, the thin plate clutch ring improves the weight saving and the resources saving, thereby providing an excellent environmental advantage. In addition, a surface area of the clutching section of the clutch ring and a pith and a torsion angle (corresponding to a lead angle of the reversible thread) of plate teeth may easily be formed as desired regardless of the thickness of the thin plate, thereby increasing the degree of freedom in a layout of the automatic stroke adjustment device.
Embodiments of this invention is applicable to a brake actuator in which the automatic stroke adjustment device is to be installed at an opening in a cylinder bore of a hydraulic operating means or mechanical operating means, thereby providing a great applicability.
Embodiments of this invention is applicable to a brake actuator in which the automatic stroke adjustment device is to be installed at the bottom side of the cylinder bore.
In addition, an automatic adjusting member can simply be a thermo deformable material such as a bimetal and a shape memory alloy so that the clutch engagement of the clutch ring may be released when the brake temperature reaches the predetermined degree, thereby eliminating an occasion for damaging the automatic stroke adjustment function. For example, over adjustment of the shoe clearance when the brake drum is expanded due to the heat may be prevented, and an operation stroke of the brake actuator under the normal temperature may be reduced.